A guest-host liquid-crystal display element employing a guest-host liquid-crystal composition obtained by mixing a host liquid-crystal composition with one or more dichroic dyes is receiving attention as a reflection type or another display element for use in future potable information apparatus, because of its superiority in viewing angle, brightness, etc.
The display qualities, such as contrast, of this guest-host liquid-crystal display element directly correlate with the dichroism or order parameter of the guest-host liquid-crystal composition used. Hence, the guest-host liquid-crystal composition has hitherto been required to have a high order parameter. In particular, a guest-host liquid-crystal composition having a high order parameter is desired recently which employs a new host liquid-crystal composition such as a fluorine-containing liquid-crystal composition and is usable in a thin-film transistor type liquid-crystal display element (TFT-LCD) and the like.
Improvements of such guest-host liquid-crystal compositions in dichroism or order parameter should be conducted with respect to both of dichroic dyes and host liquid-crystal compositions. However, the host liquid-crystal compositions which have. conventionally been investigated or improved in dichroism or order parameter have been substantially limited to those containing as a major component an Np liquid-crystal composition having a bicyclic skeleton structure such as a biphenyl or cyclohexylphenyl skeleton having a cyano group, as described later.
Consequently, in the case of using new liquid-crystal compositions such as the aforementioned fluorine-containing liquid-crystal compositions, conventional common sense does not always hold and the requirements which such a host liquid-crystal composition must satisfy have been unknown. This has made it difficult to develop a guest-host liquid-crystal composition having a high order parameter which contains a fluorine-containing Np liquid-crystal composition or Nn liquid-crystal composition.
The reasons for the above are as follows. Effects of the components and compositions of host liquid-crystal compositions on the dichroism or order parameter of guest-host liquid-crystal compositions were initially investigated with respect to liquid-crystal compositions of the Schiff base type, ester type, or the like. Thereafter, investigations were directed mainly to liquid-crystal compositions containing as a major component a bicyclic Np liquid-crystal composition having a cyanophenyl structure as a skeleton structure, such as Np liquid-crystal compositions of the biphenyl or phenylcyclohexane type having a cyano group in the direction of the molecular axis.
In the above investigations, there was an idea that the nematic/isotropic transition temperature (TNI) of a host liquid-crystal composition employed in a guest-host liquid-crystal composition should be used as an index, because the dichroism or order parameter of the guest-host liquid-crystal composition must depend on the order parameter of the host liquid-crystal composition itself used, and because the order parameter of the host liquid-crystal composition itself must depend on the thermal stability of the liquid-crystal composition. Namely, it was presumed that a guest-host liquid-crystal composition containing a host liquid-crystal composition having a high TNI generally shows high dichroism or a high order parameter.
However, the above presumption does not hold for host liquid-crystal compositions having different makeups, although almost applicable in the case of using host liquid-crystal compositions having similar makeups. Therefore, the TNI of a host liquid-crystal composition has no correlation with the order parameter thereof.
For example, it was reported in Preprints of the 9th Liquid Crystal Symposium, p. 159 and in Mol. Cryst. Liq. Cryst., 1993, Vol. 225, pp. 197-210 that order parameter is often far more influenced by structural differences between liquid-crystal components. Furthermore, there are cases where even when the same dichroic dyes are used, liquid-crystal compositions having a low TNI have a higher order parameter than liquid-crystal compositions having a high TNI (see Example 6 and Comparative Example 4 which both will be given later in this specification). It became apparent from the above that the use of TNI as an index for various kinds of liquid crystals is difficult.
Thereafter, investigations on order parameter came to be directed to guest-host liquid-crystal compositions containing as a host liquid-crystal composition the aforementioned bicyclic Np liquid-crystal composition of the biphenyl or phenylcyclohexane type having a cyano group. Specifically, examples of such cyano liquid-crystal compositions containing bicyclic ingredients as major components include liquid-crystal compositions ZLI-1132 and E-7 (both are trade names) manufactured by E. Merck, which have the structures and compositions shown below. This is because the use of these cyano liquid-crystal compositions as host liquid-crystal compositions was found to be effective in imparting not only high contrast but excellent reliability with regard to light stability, thermal stability, storage stability, etc. Thus, the host liquid-crystal compositions used came to be substantially limited to these cyano liquid-crystal compositions. Later investigations shifted to those intended for structural improvements of dichroic dyes.
______________________________________ (Composition of ZLI-1132) ##STR1## 24 mol % ##STR2## 36 mol % ##STR3## 25 mol % ##STR4## 15 mol % (Composition of E-7) ##STR5## 51 mol % ##STR6## 25 mol % ##STR7## 16 mol % ##STR8## 8 mol % ______________________________________
However, from an investigation on the re-entrant phenomenon of liquid-crystal phases and from other investigations, the above kind of Np liquid-crystal compositions having a cyano group were thereafter perceived again to have intense microscopic association among liquid-crystal molecules. Furthermore, reports were made successively on the possibility that these liquid-crystal compositions might have strong microscopic interaction with specific dichroic dyes, in particular anthraquinone dyes. As described above, the conventional investigations on dichroism or order parameter have been partial and limited to those using special liquid-crystal systems containing cyano liquid-crystal compositions and showing intense association.
Currently, the liquid-crystal compositions for use in active drive for TFTs or the like have come to frequently employ compounds having a tricyclic skeleton, such as phenylcyclohexylcyclohexane, as shown in The Chemical Society of Japan, Kikan Kagaku Sosetsu (The Elements of Chemistry, Quaternary Publication) "Ekisho No Kagaku (Chemistry of Liquid Crystal)", No. 22 (1994), p. 48. These tricyclic compounds have conventionally been scarcely used because they are disadvantageous in that a smectic phase is apt to appear or too high a viscosity results. However, since these tricyclic liquid-crystal compositions for use in TFTs and other applications show weak association with dyes, there has been no guest-host liquid-crystal composition which contains such a polycyclic liquid crystal and shows high dichroism or a high order parameter.
Guest-host liquid-crystal compositions containing new host liquid-crystal compositions such as fluorine-containing liquid-crystal compounds have a problem that they often are considerably inferior in the dichroism of anthraquinone dyes, many of which are included in important blue dyes, to guest-host liquid-crystal compositions containing the above-described Np liquid-crystal compositions having a cyano group. Because of the above problem, the idea that the cyano liquid-crystal compositions described above have strong microscopic interaction with anthraquinone dyes and thus greatly influence high dichroism came to be supported.
Thereafter, as a result of trials and errors, the above-described decrease in dichroism of anthraquinone dyes was found to be relatively small in fluorine-containing liquid-crystal composition ZLI-4792 (trade name, manufactured by E. Merck) among other fluorine-containing liquid-crystal compositions. Although the liquid-crystal composition available under the above trade name has come to be used as a common fluorine-containing host liquid-crystal composition as shown, e.g., in Examples of JP-A-7-179858 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), the above prior art liquid-crystal composition is still insufficient in dichroism and contrast.
For example, the anthraquinone dye represented by the following structure: ##STR9## has a relatively high order parameter of 0.78 when used. in combination with cyano liquid-crystal composition ZLI-1132, manufactured by E. Merck and having the structures and composition shown above, but has a considerably reduced order parameter of 0.73 when used in combination with fluorine-containing liquid-crystal composition ZLI-4792 (product of E. Merck).